In a mobile phone, fingerprints can only be identified and processed after a fingerprint image is captured by a fingerprint capture device. The quality of the fingerprint image directly affects the identification accuracy and the processing speed of a fingerprint identification system. Therefore, fingerprint capture is a key technology for fingerprint identification.
Main fingerprint capture technologies used nowadays include an optical fingerprint capture technology, a semiconductor fingerprint capture technology and an ultrasonic fingerprint capture technology.
Apple uses a silicon capacitive fingerprint image sensor, which is the most common semiconductor fingerprint sensor (semiconductor fingerprint capture technology), for capturing fingerprints through electronic scales. About 100,000 capacitive sensors can be combined on a semiconductor metal array with an insulating outside surface. Each point of the sensor array is a metal electrode that acts as an electrode of a capacitor, a corresponding point of a fingertip on a sensing surface serves as the other electrode, and the sensing surface forms a dielectric layer between the two electrodes. Since the distances between a groove and a valley of the fingerprint and the other electrode are different (the presence of the depths of lines), a value of each capacitance of a silicon surface capacitance array is different, and the capacitance value of each point is measured and recorded, so that a fingerprint image with gray scale may be obtained.
A fingerprint scanning apparatus and a fingerprint identification chip correspond to the optical fingerprint capture technology, the fingerprint identification chip is connected to a mobile phone circuit board by wire signals. The optical fingerprint capture technology is the oldest and most widely used fingerprint capture technology currently, and the principle is the total reflection of light. Fingerprints have grooves and valleys. After light is irradiated to the valley by glass, total reflection occurs at an interface between the glass and air. The light is reflected to a Charge Coupled Device (CCD), and total reflection does not occur to the light that is directed at the groove, but is absorbed by a contact surface of the groove and the glass or diffused to other places. Thus, a fingerprint image is formed on the CCD.
The ultrasonic fingerprint image capture technology has not been applied to a fingerprint identification system because of the high cost.
Table 1 shows a comparison of the three main technologies.
UltrasonicOptical scanningSemiconductorscanningtechnologysensing technologytechnologyImagingA dry finger has aA dry finger has aVery goodcapabilitypoor imaginggood imagingcapability, andcapability, andimaging of a fingerimaging of a fingerthat is sweaty andthat is wet and roughswollen is fuzzy.is achieved. TheThe imagingimaging capability iscapability is easilyeasily affected byaffected by dirt anddirt and grease ongrease on skin.skin.Imaging areaLargeSmallMediumResolutionLower than 500 dpiUp to 600 dpiUp to1000 dpiDevice sizelargesmallMediumDurabilityVery highHighMediumPowerHighLowHighconsumptionCostHighMediumVery high
As can be seen from Table 1, the above three methods have high costs, and application to a mobile phone will undoubtedly increase the sales price of the mobile phone, which is not conducive to mobile phone sales.